Immunizing mice with preformed antigen--antibody complexes is a highly effective means of generating B-memory cells. In the present study we have compared the capacity of mouse anti-dinitrophenyl (DNP) IgM, IgG1, IgG2 and IgA antibodies to generate DNP-specific memory, when given in complex with antigen (DNP-KLH: keyhole limpet haemocyanin). Two monoclonal IgM antibodies exerted no adjuvant effect, whereas a monoclonal IgA antibody was effective, IgG2 antibodies were a more powerful adjuvant than IgG1, regardless of whether anti-DNP or anti-KLH antibodies were used. Furthermore, the capacity of DNP-KLH--antibody complexes to localize in splenic lymphoid follicles could be ranked IgG2 greater than IgG1 greater than IgA; IgM complexes did not localize in follicles. These results correlate well with data (presented elsewhere) on the capacity of these different antibodies to activate mouse complement, and confirm that C activation is an essential requirement for both follicular localization of immune complexes, and for the generation of B-memory cells. Although activation of the alternative complement pathway is sufficient to effect both processes, the results with IgG2 antibodies raise the possibility that classical pathway activation may be more effective.